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Old 07-04-2022, 06:31 PM   #1
City: Berkeley
Join Date: Aug 2019
Posts: 17
Solar Thermal Hot Water

Have any Forum members looked carefully at the relative energy, $ cost, and tradeoffs of solar thermal hot water systems versus the alternatives for vessels (like us) not on shorepower and at anchor or moored, not underway most of the time (and therefore not using the motor heat exchangers to heat the hot water system (HWS)?
Ocean Alexander Aquabelle has motors and AC from generator to heat a 1.8kWe element in an 81 liter HWS tank in the engine room. But most of the time, we don't use the motors as we are moored. Our current options are:
Genny: To go from 10 to 70 degrees C takes about 4-5 kWhe and about 1.5 hours running the genny which uses about 4+ liters of diesel--a highly inefficient use of the fuel when heating water is the primary reason to run the genny.
Photovoltaics (PVs): Aquabelle now has 2.4 kWe of solar which generates substantially more power on a sunny day than needed for house use. We can switch on the HWS circuit breaker to heat the HWS via the inverter but a 50A solar current immediately shifts to a -70 to 80A drawdown of the fully charged batteries (960A total storage). So excess solar can handle some but not all of the hot water heating, depending on how much energy remains in the HWS from the day before, in turn dependent on insulation and how much cold water has been introduced by day-use of hot water. At the moment, we have to remember to turn off the HWS circuit breaker, but we could (like Brian on Insequent) install a 2-3 hour timer on the circuit to avoid running the housebank too low (provided we check the voltage before turning on the HWS charging at the breaker to check the housebank is fully or nearly fully charged by the solar). One alternative is to add more solar and/or more batteries, but that's constrained by real estate as the flybridge is now fully occupied by six solar panels (including one slide out panel from under the radar arch that extends aft over the deck about 0.4 meters). And, in addition to substantial front end cost, albeit zero fuel cost, more PV will not deliver in shitty weather like we are having right now as yet another cataclysmic storm smashes NSW coast (this time south of here, in Sydney and surrounds). But then the genny can be used via AC circuit at fuel-only marginal cost...
3. A diesel heater such as Webasto hydronic heating system via an HX loop on Brian's Insequent: this has the co-benefit of offering space heating if one can run the hydronic distribution piping and cabins have accessible space for the radiators); and is likely pretty efficient in using the diesel--my guess is that the system would run at 70-80% delivery of diesel energy content to thermal value of hot water versus using the genny to deliver energy via the AC circuit--which is perhaps 15 max 20% efficient.
Overall: Fairly substantial front end cost, on-going diesel cost and covers cloudy periods w/o further backup needed although genny is always available at higher diesel cost (and noise, fumes...).
4. Last but not least, and the reason for this post: solar thermal. I have never understood why solar thermal isn't built into production boats, just as PVs are starting to be integrated from get-go either into the topsides or on all sorts of custom mounts seen today on motor vessels and yachts. Obviously, one needs the real estate to mount--but because they collect 80% or more of the solar energy hitting the boat versus max 30-40% for PVs, the footprint is much smaller. At the moment, there are few "marinized" units with fittings suitable for the caustic environment of coastal moorings or at sea, but there are some. I haven't seen any parabolic units on the market with the collector inside a protective cover, but some exist in R&D phase and these are inherently more efficient collectors and could be pole mounted or mounted on a radar arch along with a tracking device. The circulating pump can be solar powered itself w/o a battery that operates at a speed determined by the intensity of the incoming solar energy; or a circulating pump located below decks, presumably in an engine room, powered by an inverter AC circuit or by DC off the housebank.
Here's one such unit produced in the US more for the RV market than the marine market, see
but they make units for marine use specifically
and marketed in Australia
and here's one report from Qld who installed on a yacht before the local distributor setup shop
The units use plumbing for US sized pipes and threads, so that complicates things slightly (the Australian distributor can supply all the substitute parts so you don't have to go searching for the supplier and do all the conversions to get the right units).
Some installation considerations including running the insulated lines to/from the HWS--a 20 meter round trip for hot/cold water on Aquabelle; shading losses just like PVs; and ugliness of visible plumbing on deck or in profile.
Overall, the tradeoffs are the front end cost; finding the real estate (about 2m2 or 4 panels for the Heliatos system sized to our 81 liter HWS); no fuel cost; intermittency of sun so still needs backup for shitty weather, just like the additional PV option, although one could still rely on the genny for these periods.
So there we have it: 4 options at anchor or no shorepower and not under way.
1. Business as usual, run the genny solely to heat water at marginal cost of fuel, some noxious fumes and noise;*
2. Add PV to existing PV system to gain more excess power and rely on genny for top-up or backup heating. (Problem, we have no easily available real estate for more PV).
3. Install a diesel heater, substantial front end cost, always works, but uses diesel albeit about 25% of using genny, so that's a saving relative to BAU; only needs space in ER, no need for deck space to mount but will need it's own exhaust system with less but still present noxious fumes*; has the co-benefit that it can also be used for space heating, but that cost could be avoided at least for distribution components if hot water is sole goal).
4. Install solar hot water, similar front end cost to 3 but no fuel cost, but does need deck space; and like 2, still needs genny as backup for intermittent sun; but when combined with existing PV to preheat the water, may do the trick most days even in poor solar conditions to minimize backup or top-up genny use.

* an additional consideration; we are moored angled NE to SE at Evans Head. So unlike swinging on an anchor where one is usually head to wind so exhaust streams downwind and away from the boat. cross winds can bring fumes back to the boat and require closure of portholes and windows/doors while genny runs. In summer, this requires turning on the AC while genny runs!

Do any forum members have experience with solar thermal systems on their boat?
Has anyone installed a Heliatos or know of similar units?

Photos of PVs (before the sixth after slide-out panel installed) on hard top are attached for those interested.
Attached Thumbnails
PXL_20220621_014901750.jpg   PXL_20220621_014835162.jpg   PXL_20220621_072734680.jpg  
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Old 07-04-2022, 08:09 PM   #2
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City: Between Oregon and Alaska
Vessel Name: Charlie Harper
Vessel Model: Wheeler Shipyard 83'
Join Date: Jun 2016
Posts: 2,733
My hot water tank is heated by a coil in my diesel stove. Or I can use electricity from a generator or inverter.

My tank is super insulated. It has 2" foam top, bottom, and sides. Also the cold water line is insulated for about 6' so it doesn't draw heat from the tank. I have a 50 gallon tank and can go several days with hot water when on the boat alone. I usually run the generator when there are several needs like charging inverter bank, making water, laundry, dishwasher.
Solar I've thought about. I just paid $6.75 for diesel as an incentive.
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Old 07-04-2022, 09:09 PM   #3
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City: Litchfield, Ct
Join Date: Aug 2012
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I have a full time cruising friend who installed a solar hot water heater, probably the Heliotose direct HW system.

On a sunny day it would heat up the HW tank to the low 100s F and on a cloudy day not so much. The solar collector was quite small, about 12 x 18”.

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Old 07-04-2022, 10:00 PM   #4
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City: Queensland
Vessel Model: Milkraft 60 converted timber prawn trawler
Join Date: Jul 2016
Posts: 4,357
Going through similar to the op ourselves
We have 140 litres with a 1.8kw element but don't need anything like that amount of hot water
Washing up daily and yachtie showers every 2nd day
In summer, cold showers are the norm

Usually heating from 46c to 60c takes around 2 hours, a lot of power for washing up that would be better used for charging big Lifepo4 bank, running the still, washing machine, vacuum etc.

We also have 2.5 kW of solar and, on a good sunny day we turn the HWS on around 10am to be done by 12:00
During summer it uses very little battery but winter, especially this year with grey and rain predominately, its a mainly Genset thing so an actual solar HWS will be of little use

Instead what we will likely do is get rid of the 140 litre HWS and replace with a 25 litre version of the same.
10 minutes to go from 46c to 60c or half an hour from 20c cold to 60c using the standard 2.2kw element so says this calculator

$400 for the electric vs $800+ for the solar.
Everything on a boat is broken, you just don't know it yet
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Old 07-05-2022, 01:16 AM   #5
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City: Brisbane
Vessel Name: Insequent
Vessel Model: Ocean Alexander 50 Mk I
Join Date: Mar 2012
Posts: 3,673
I've seem some houseboats with "instantaneous propane" style HWS. Both the cylinder and heater module would need to be externally mounted. For that matter an external propane heater could likely be plumbed into an existing HWS that is in the ER (or wherever).

Something like this for example
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Old 07-05-2022, 07:36 AM   #6
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City: Litchfield, Ct
Join Date: Aug 2012
Posts: 6,331
Let me analyze physics to explain why direct solar hot water panels are more efficient than solar electric panels:

If you use solar electric panels for heating your hot water the whole process is less than 20% efficient. The solar panel itself at best converts 22% of the sun's energy into electricity. Then you have controller losses, plus inverter losses which brings the overall efficiency well below 20%.

For direct solar heating panels there are thermal losses at the panel itself- heat dissipated from the panel, heat losses in the tubing down to the heat exchanger and energy losses in the pump that circulates the hot water. I am guessing that this all adds up to about 50% of the solar input, but that is just a guess.

So, if correct a direct solar heating panel will perform 2.5 times (50%/20%) better than solar electric heating. Or said another way, if a 1 square foot panel works for direct solar heating, it will take 3 square feet of panels to do the same with solar electric.

That is consistent with my experience on my friend's boat where he has the capability to do both.

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Old 07-05-2022, 08:31 AM   #7
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City: Fort Myers, FL... Summers in the Great Lakes
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 11,074
Regarding generator use:
I've seen several boats where a heat loop off the generator engine was plumbed into a water heater heat exchanger. I know of atleast one water heater manufacturer that offers 2 heat exchangers in their water heaters. So you could use one on the main engine and one on the generator. I would imagine that the recovery time for a water heater utilizing both generator electricity and the coolant loop, would be much faster.

Regarding Solar Thermal:
If I wanted to MacGyver a solution, I would contact a swimming pool solar heating panel manufacturer. Generally the panels are 2' wide with varying lengths. I'm guessing a 2' x 4' panel would do a great job of heating a 20 gallon water heater tank.

I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
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Old 07-05-2022, 11:49 AM   #8
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City: Aventura FL
Vessel Name: Kinja
Vessel Model: American Tug 34 #116
Join Date: Oct 2017
Posts: 9,629
Two points
1. put an anti-scald valve in the system. Even the small solar heaters are very efficient.
2. gotta have a water maker too.
The meek will inherit the earth but, the brave will inherit the seas.
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